Internet of Things (IoT) is expected to increase the number of connected devices significantly. A vast majority of these devices will likely operate in unlicensed radio bands, in particular in the 2.4 GHz Industrial, Scientific and Medical (ISM) radio band. At the same time, there is increased demand for using the unlicensed radio bands also for services that traditionally have been supported in licensed radio bands. As an example of the latter, third Generation Partnership Project (3GPP) that traditionally develop specifications only for licensed radio bands have now also developed versions of Long Term Evolution (LTE) which will operate in the 5 GHz unlicensed radio band.
Technologies that are expected to dominate for IoT services are Bluetooth Wireless Technology, in particular Bluetooth Low Energy (BLE), and future versions of the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard, like the IEEE 802.11ax standard.
The IoT applications are foreseen to most often have rather different requirement and features compared to applications like e.g. file download and video streaming. Specifically, the IoT applications would typically only require low data rate and the amount of data transmitted in a single packet may often only be a few bytes. In addition, the transmissions to and from many devices will occur very seldom, e.g. once an hour or even less often. The number of IoT devices is expected to be huge, which means that although the amount of data to each one of the devices may be small, the aggregated IoT data may still be substantial. Many use cases for IoT applications can be found in an ordinary house, and may be related to various sensors, actuators, etc. The requirements for coverage are therefore substantially less than what usually can be achieved by e.g. a cellular system. On the other hand, the coverage which can be obtained by e.g. the Bluetooth or the IEEE 802.11b/g/n/ac technologies may not suffice. This may be in particular true if one of the devices is outdoors whereas the other device is indoors so that an exterior with rather high penetration loss is in between the devices.
The straight-forward approach to increase the range of a communication link is to reduce the bit rate that is used. Reducing the bit rate by necessity means that it will take longer to transmit a packet of a certain size. As a side effect of this, the channel will be occupied for a longer time. Now, with a large number of devices sharing the same channel, the channel may be congested if this sharing is not done in an effective way. The need for long packets and the increased number of users will make this congestion even more pronounced.
Moreover, the amount of non-IoT data, e.g. data download and video streaming, sent over the same channel may also increase. This implies that to obtain good performance for both the IoT applications and the non-IoT applications, some coordination should preferably take place.
An obvious, and probably the simplest, way to do such coordination is by time sharing between the IoT system and the non-IoT system. However, as the data rate for the IoT system is very low for the individual links, it may likely be hard to obtain good spectrum efficiency in this way. Instead it would be preferable if the two systems, i.e., both the IoT system and the non-IoT system could operate concurrently. One means to achieve this could be if the non-IoT system would be based on Orthogonal Frequency Division Multiplexing (OFDM). Concurrent operation could then be achieved by assigning one or more sub-carriers to the IoT system and the remaining ones to the non-IoT system. The number of sub-carriers allocated to the IoT system could in this way be rather flexible.
Bluetooth and GFSK
Variants of Frequency Shift Keying (FSK) are used in e.g. Bluetooth Wireless Technology. The FSK is a frequency modulation wherein digital information is transmitted through discrete frequency changes of a carrier signal. The Bluetooth technology employs Gaussian Frequency Shift Keying (GFSK). The GFSK is a constant envelope modulation which allows cost efficient implementations. At the receiver side, a simple limiting receiver may be used, i.e., the Analog-to-Digital Converter (ADC) may be replaced by a simple comparator and there will essentially be no need for an Automatic Gain Control (AGC) in the receiver, further simplifying the implementation and reducing the cost. Even more significant is the gain at the transmitter side. Due to that the GFSK is a constant envelope modulation, there is less need to back-off the Power Amplifier (PA) and there are much less linearity requirements on the PA, and thereby significantly higher power efficiency can be obtained. The OFDM is known to suffer severely from a high Peak-to-Average-Ratio (PAR), which means a less efficient transmission than FSK. Since an IoT device, such a sensor, may be powered by a coin battery, the power efficiency of the device is one of the key features.
OFDM
A block diagram for an OFDM transmitter is shown in FIG. 1. First the information is processed by an Inverse Fast Fourier Transform (IFFT), which effectively transforms the signal from the frequency domain to the time domain. After that a Cyclic Prefix (CP) is added. Then, the signal is passed through a Digital-to-Analog Converter (DAC), after which it is up-converted in frequency to the carrier frequency. This up-conversion is what in FIG. 1 is referred to as the mixer. Finally, the signal is amplified by means of the PA before it is transmitted.
Non-Orthogonal Multiple Access Via Overlay/Underlay
It is apparent from the discussion above that concurrent operation of narrowband (NB) and wideband (WB) stations, e.g. NB and WB devices, may be advantageous in wireless networks supporting both IoT applications and high data rate applications. Due to the power efficiency and cost, it is desirable to allow NB devices supporting GFSK. This can be achieved by introducing a non-orthogonal multiple access technique which we shall name overlay/underlay. FIG. 2 schematically illustrates a transmitter implementing the overlay/underlay technique. It is seen how a GFSK signal can be added to an OFDM signal prior to performing the DAC, up-conversion by means of the mixer, and signal amplification by means of the PA. Frequency domain multiplexing of OFDM signals intended to WB STA's and NB GFSK signals intended for NB STA's can be achieved by simply not transmitting, i.e. blanking, the WB signals on some of the sub-carriers, i.e., effectively setting the corresponding frequency bins to zero in the IFFT, as illustrated in FIG. 2. This will result in a “gap” in the spectrum. This gap can be placed where desired by simply setting the corresponding sub-carriers to zero. The NB signal can then be assigned to the nulled OFDM sub-carriers. The NB GFSK signal can then be added to the WB OFDM signal, by placing it in the above mentioned generated gap. One means to achieve this is to generate the NB GFSK signal at baseband, and then just shift it in frequency so that it fits in the gap.
Although some of the sub-carriers are not used by the high data rate signal, it does not mean that placing a low-rate signal in the gap will ensure that the signals do not interfere with one another, i.e., they will not necessarily be perfectly orthogonal. We say that the NB signal(s) is overlaid over the WB signal(s), and called the NB signal an overlaid signal and the WB signal an underlaid signal. This non-orthogonal multiple access scheme is named overlay/underlay. Even though it has been described only with one overlaid signal, it is straightforward to generalize it to two or more overlaid signals.
Although overlaying GFSK signals over an underlaid OFDM/OFDMA signal may give acceptable results, link performance is not so good.